1. Field of the Invention
This invention relates to a light transmitting/receiving module, eg., to an LD/PD (Laser Diode/Photodiode) module, an LD/APD (Laser Diode/Avalanche Photodiode) module, an LED/PD (Light Emitting Diode/Photodiode) module or an LED/APD (Light Emitting Diode/Avalanche Photodiode) module unifying a light transmitting device and a light receiving device which are used at base ports (broadcasting station) and subscribers in a unidirectional or bidirectional optical communication system capable of transmitting optical signals with different wavelengths in a unidirectional or bidirectional manner, and more particularly, relates to an LD/PD or an LED/PD module enjoying a simple structure, easy production method, high reliability, and low cost.
2. Description of Related Art
This application claims the priority of Japanese Patent Application No. 11-38672 (38672/1999) filed Feb. 17, 1999 which is incorporated herein by reference.
[Explanation of Bidirectional Optical Communication]
Recent development of technology has been striving to reduce the transmission loss of optical fibers and has been enhancing the properties of semiconductor laser diodes (indicated as LDs hereafter) and semiconductor photodiodes (indicated as PDs hereafter).
The light emitting devices used in the optical communication system include LDs and LEDs, but in this invention an LD (laser diode) is picked as a light emitting device. Further, the light receiving devices includes PDs and APDs, but this invention picks a PD (photodiode) as a light receiving device. This invention can include both the LD and the PD. There exist telephones, facsimiles, televisions and so on as means for transmitting information. Particularly, people have rigorously tried optical communication utilizing long wavelength light, for example, the light with a 1.3 xcexcm wavelength or the light with a 1.55 xcexcm wavelength. Recently, there develops a bidirectional transmission system capable of transmitting and receiving signals in forward and backward directions at the same time by only a single optical fiber. Such a communication system is called a xe2x80x9cbidirectional communication systemxe2x80x9d. Single optical fiber is one of the most beneficial advantages.
FIG. 1 schematically shows a multiwavelength bidirectional optical communication system which adopts a plurality of wavelengths for transmitting signals simultaneously both in a forward direction and in a backward direction. One station is connected to a plurality of subscribers (ONUs) by optical fibers. Although FIG. 1 shows only a single subscriber for simplicity, many subscriber ports are connected to the central station. The fiber from the station branches through many bisecting points into a plurality of fibers linking with individual subscribers.
The central station amplifies the signals of telephones or televisions as digital signals or analog signals, and drives a semiconductor laser (LD1) which produces xcex1 light responsive to the amplified signals. The light of xcex1 emitted from the LD1 enters an optical fiber 1 as light signal of xcex1. A wavelength division multiplexer (WDM) 2 introduces the xcex1 light into an intermediate optical fiber 3. Another wavelength multiplexer (WDM) 4 allocated to the subscriber side leads the xcex1 light to an optical fiber 5, and the xcex1 light is received by a photodiode (PD2) capable of converting the optical signals to electric signals. A receiver apparatus on the subscriber side amplifies and processes the electric signals for reproducing the voice of telephone or the image of television. The signals flowing from the station to the subscribers are called xe2x80x9cdownward signalsxe2x80x9d, and the direction is called a xe2x80x9cdownward directionxe2x80x9d.
On the contrary, the subscriber converts electric signals of facsimile or telephone into xcex2 light signals by a semiconductor laser diode (LD2). The xcex2 light enters an optical fiber 6, is introduced to an intermediate optical fiber 3 by the WDM 4, and enters the PD1 capable of converting the xcex2 signals to electric signals, passing through the WDM 2. These electric signals are dealt with by converters or signal processing circuits on the station side. The signals flowing from the subscribers to the station are called xe2x80x9cupward signalsxe2x80x9d, and the direction of the signals is called an xe2x80x9cupward directionxe2x80x9d.
The above system appropriates xcex1 to the downward signals, and xcex2 to the upward signals exclusively. In practice, light with the same wavelength may be used for both the upward and downward signals. Another system allocates two wavelengths xcex1 and xcex2 to both the upward signals and the downward signals. In this case, the separation of two wavelengths is an extremely important problem in the optical communication system capable of carrying two different wavelength signals in an optical fiber. [Explanation of Wavelength Division Multiplexer]
When the bidirectional communication (two-way communication) using two sorts of light with different wavelengths is carried out by only one optical fiber, both the station and the subscribers require a device for discriminating and separating the wavelengths. The WDMs 2 and 4 play the role of distinguishing and separating different wavelengths. The WDMs has the function of connecting the xcex1 light to the xcex2 light for leading both lights into a fiber, and the function of extracting only one wavelength light from two sorts of wavelength light propagating in the fiber. Therefore, the WDMs perform an important part in the multiwavelength bidirectional communication systems.
Various types of wavelength division multiplexers have been proposed, which will be explained by FIG. 2 and FIG. 3. FIG. 2 indicates a WDM consisting of optical fibers or optical waveguides. Two optical paths 8 and 9 are close to each other at a part 10 for exchanging the energy of light therebetween. Various coupling modes are realized by determining the distance D and the length L of the close part 10. For example, when xcex1 enters the optical path 8, the light with xcex1 wavelength appears in a path 11. When xcex2 enters an optical path 12, the light with xcex2 wavelength appears in the optical path 9 instead of the path 8.
FIG. 3 shows another WDM that uses a multilayered mirror. This WDM consists of two rectangular isosceles triangle columns (glass blocks) 13 and 14 and a dielectric multilayer mirror 15 formed on the slanting plate of the columns. It is possible to make the whole of xcex1 light pass through the dielectric multilayer mirror and to make the whole of xcex2 light reflect from the dielectric multilayer mirror by an appropriate combination of the refractive index and the thickness. The dielectric multilayer allows the incident light at 45 degrees to pass through or reflect. This dielectric layer type WDM can be utilized for the WDMs 2 and 4 in the optical communication system of FIG. 1. The WDM is sometimes called a xe2x80x9cwave-division-integration devicexe2x80x9d or xe2x80x9cwavelength division multiplexerxe2x80x9d. Fiber-type WDMs and glass block type WDMs have already been on sale.
An example of an LD/PD module on the subscriber side is explained by referring to FIG. 4. A single mode optical fiber 16 leading from the center station to the subscriber side is connected to an optical fiber 18 of a subscriber (ONU) module by an optical connector 17. The ONU module has a fiber-type WDM 21 which couples the fiber 18 to a fiber 19 with wavelength-selectivity An optical connector 22 couples the optical fiber 18 to an LD module 25 in the ONU. Another optical connector 23 joins the fiber 19 to a PD module 27.
The light signals transmitted from the LD 25 via the optical fibers 24 and 18 are upward system. 1.3 xcexcm light carries signals from the subscriber side to the station through the upward system. The signals transmitted from the fibers 19 and 26 to the PD module 27 are downward system. 1.55 xcexcm light carried from the station is converted to electric signals by the PD module 27. The LD module 25, which is a signal transmitting device, includes an electric circuit for amplifying and modulating the signals of telephones and facsimiles, and a laser diode (LD) for converting electric signals into optical ones. The PD module 27, which is a receiving device, contains a photodiode for converting optical signals from the station into electric signals, an amplification circuit for amplifying the optical signals and a demodulation circuit for restoring the television signals or telephone signals. The WDM 21 has the function of separating the 1.55 xcexcm light from the 1.3 xcexcm light. This example allots the 1.3 xcexcm light to the upward system and the 1.55 xcexcm light to the downward system.
This invention provides an improved LD/PD module for bidirectional multiwavelength optical communication. The LD/PD module contains a light emitting device, a light receiving device, circuits around these devices, and so on. Related art will be explained on every device.
[Explanation of Conventional Semiconductor Laser Diode (FIG. 5)]
FIG. 5 shows a conventional laser diode device 25, which includes a semiconductor laser diode (LD) chip 29 and a monitoring photodiode (PD) chip 30. The laser diode chip 29 is vertically mounted on a side surface of a protrusion part (pole) 31 on a header 32. The light emitted from the laser diode chip 29 is in parallel to the chip surface. The photodiode chip 30 is mounted on the top surface of the header 32 at a spot where the backward light emitted from the laser diode chip 29 attains. A plurality of lead pins 33 are implanted on the bottom of the header 32. A cap 34 covers the top surface of the header 32.
The cap has a window 35 at the center. The light beams emitted from the laser diode 29 go out in both upward and downward directions. There is a lens 37 fixed just above the window 35 and supported by a lens holder 36. A conical housing 38 covers the top of the lens holder 36. A ferrule 39 is fixed to the top part of the housing 38. The ferrule 39 holds an end of an optical fiber 40. The ends of the ferrule 39 and the fiber 40 are polished at a slanting angle of about 8 degrees so as to prevent reflected light from going back to the laser. The holder 36 is aligned at an optimum spot to the header 32 by sliding the holder 36 and measuring the light power from the semiconductor laser at the other end of the fiber 40. Wires connect the pads of the laser diode chip 29 and the photodiode chip 30 with the lead pins 33, respectively.
The lens 37 converges the light beams emitted from the laser 29 on the end of the fiber 40. Since the laser is modulated by a driver circuit with an electric signal, the light carries the signal. The output of the laser diode is monitored by the monitoring photodiode 30. The material of the laser determines an oscillation wavelength ranging from 1.3 xcexcm to 1.55 xcexcm.
[Explanation of Conventional Photodiode Module (FIG. 6)]
FIG. 6 shows an example of a conventional photodiode module. A photodiode chip 41 is die-bonded on the upper surface of a header 42. There are lead pins 43 on the bottom surface of the header 42. A cap 44 covers the top surface of the header 42. An opening 45 is perforated at the center of the cap 44. A cylindrical lens holder 46 encircles the cap 44 upon the header 42, and supports a lens 47.
The lens holder 46 has a conical housing 48 on the top. A ferrule 49 grips an end of an optical fiber 50. The housing 48 holds the ferrule 49. The ends of the ferrule 49 and the optical fiber 50 are slantingly polished for suppressing reflected light from going back to the laser diode.
The holder 46, the housing 48 and the ferrule 49 are aligned at optimum spots by penetrating light to the fiber and measuring the light power from the photodiode chip 41. The material of the light receiving layer of the photodiode 41 determines the range of a wavelength of light detectable to the photodiode device. A silicon (Si) photodiode is available for sensing visible light. Such a Si photodiode is, however, irrelevant for the present invention which aims at an LD/PD module intended for infrared light. Sensing of infrared light needs a semiconductor photodiode having an InP substrate enjoying a narrow band gap and an InGaAs or an InGaAsP light receiving layer enjoying a narrow band gap for absorbing infrared light.
[Problems to be Solved]
Problems of the conventional LD/PD modules will be pointed out. Subscribers would be mostly households. Thus, the optical bidirectional communication network would acquire a great market as well as telephones being in common use now. However, consumers would not buy such an LD/PD module for optical communication unless the LD/PD module should be dropped in price as low as telephones. Since the conventional LD/PD module shown by FIG. 4 assembles the LD module, the PD module and the WDM module, it cannot be made as inexpensively as telephones. The price is the sum of the costs of modules.
Such an expensive LD/PD is an obstacle to one""s progress among subscribers. The technology of producing a low-price LD/PD is indispensable to further development of the optical bidirectional communication system. Several attempts have been practiced to produce devices enjoying a few components, compact size and low price. Some proposals for lowering the cost are now explained.
[A. Spatial Separating Beam Type Module (a Receptacle Containing a WDM, a PD and an LD)]
This paper was proposed by Masahiro Ogusu, Tazuko Tomioka and Sigeru Ohshima, xe2x80x9cReceptacle Type Bi-directional WDM Module Ixe2x80x9d, Electronics Society Conference of Japanese Electronics, Information and Communication, C-208, p208 (1996), which is shown by FIG. 7. A WDM filter 61 is centrally positioned at a slanting degree of 45 degrees in a housing 60. Three drum lenses 62, 63 and 64 are fixed on three sides of the housing 60, respectively. A photodiode (PD) 66 is mounted on one side in front of the drum lens 62. A laser diode (LD) 68 is fixed on another side in front of the lens 63. The lens 64 lies at the end connecting to an optical fiber 69.
In practice, the module consists of two separative parts. One part having the fiber end can be attached to or removed from the other part. In this case, the housing 60 and a receptacle fixing the optical fiber 69 can be easily attached or removed. The optical fiber 69 can easily be inserted to or pulled out from the housing 60. In the coupled state, an external fiber 69 is connected via the WDM 61 to the PD 66 and the LD 68. The light emitted from the optical fiber 69 is spatially expanding in the receptacle. Therefore, the lenses 62, 63 and 64 prevent the light from the fiber 69 from diverging spatially in the receptacle. The LD 68 emits 1.3 xcexcm light which penetrates the WDM filter 61 slantingly and enters the fiber 69 for sensing optical signals.
The incoming light propagating in the fiber 69 is 1.55 xcexcm light which is reflected by the WDM 61 and is transmitted to the PD 66 passing through the lens 62. The WDM filter 61 has the property of selecting wavelengths. This module is smaller size than the module shown by FIG. 4, but still includes two independent devices, e.g., the LD and the PD, and the indispensable WDM filter requiring three focusing lenses. The alignment of parts is still as difficult as the conventional module of FIG. 4. The cost is mostly the same a s the module of FIG. 4.
[B. Y-bisecting Wave Guide Type Module (FIG. 8)]
This is proposed by Naoto Uchida, Yasufumi Yamada, Yoshinori Hibino, Yasuhiro Suzuki and Noboru Ishihara, xe2x80x9cLow-Cost Hybrid WDM Module Consisting of a Spot-Size Converter Integrated Laser Diode and a Waveguide Photodiode on a PLD Platform for Access Network Systemsxe2x80x9d, IEICE TRANS. ELECTRON., VOL.E80-C, No. 1, p88, January 1997, which is explained by referring to FIG. 8. An insulating silicon substrate 70 is adopted as a base. A transparent quartz glass optical waveguide 71 is formed on the insulating silicon substrate 70. A corner of the waveguide 71 is cut to be a step part 72. Doping of impurity makes narrow Y-bisectional paths 73, 74, 76, 77 and 78 on the optical waveguide 71.
This module has two Y-branches. The first Y-branch has a WDM filter 75 at the cross point. The WDM 75 has the function of selecting wavelengths, that is, it can reflect 1.55 xcexcm light and penetrate 1.3 xcexcm light. Electrode patterns 79, 80, 81 and 82 are evaporated on the step 72 of the waveguide. An LED in this case, Edge-emitting LED (E-LED) or LD 83 having an electrode on its bottom surface is bonded on the electrode patterns 79 and 80. This is either an LED or an LD 83 capable of emitting 1.3 xcexcm light from an emitting point on an end surface.
A PD 84, which is capable of sensing 1.3 xcexcm light emitted from the end surface, is bonded on the farther electrodes 81 and 82 on the step 72. Since this module has the electrodes on the bottom surface, wire-bondings are unnecessary. This PD is novel in the type of receiving light from the end surface, but is difficult to produce due to its novelty. The PDs popularly sold on the market are not fitted to the module because of their upper surface light sensing types. The light spreading in the optical fiber 88 contains both 1.3 xcexcm light and 1.55 xcexcm light. The light goes into the path 74, and attains to the WDM 75 which reflects the 1.55 xcexcm light to another optical fiber 87. The 1.3 xcexcm light further goes ahead and enters both the Y-branched paths (waveguides) 77 and 78. The light attaining to the LED or the LD 83 is useless light. The light going to the PD 84 is detected as receiving signal light. The LED or the LD 83 makes transmitting signal light of 1.3 xcexcm which propagates in the path 78, the WDM 75 and the path 74, and enters the optical fiber 88 by converging the light with a focus lens (not shown).
This example uses the WDM only for excluding the 1.55 xcexcm light. The worst drawback of the proposed module is the difficulty of producing the planar Y-branched waveguides. The fabrication of curved Y-branch on a quartz glass waveguide layer is far more difficult than that of a straight path on a waveguide. Further, the alignment between the lens and optical fiber is difficult, because the signal light once going out to the free space must be converged into the end of the optical fiber by the focus lens. The part costs, e.g., lens and so on, increase the module price. There exists the possibility of coupling the fibers directly with the ends of the paths 73 and 74. Such a junction also requires a difficult alignment among the fibers and paths. Thus, this proposal cannot satisfy the requirement of producing an inexpensive LD/PD module.
[C. Upward Reflection WDM Type LD/PD Module (FIG. 9)]
This is proposed by Tomoaki Uno, Tohru Nishikawa, Masahiro Mitsuda, Genji Tohmon, Yasushi Matsui, xe2x80x9cHybridly integrated LD/PD module with passive-alignment technologyxe2x80x9d, 1997 Conference of Electron, Information, Communication Society, C-3-89 p198 (1997), which is explained by referring to FIG. 9. This proposal strives to satisfy the miniaturization and the enhancement by mounting an LD and a PD on a common substrate. In FIG. 9, a V-groove 91 is formed on a silicon substrate 90 by scribing the substrate with a straight line. An end of an optical fiber 92 is inserted, and is fixed in the V-groove 91. A deep slanting notch 93 is dug on the way of the optical path as being across the fiber 92 and the V-groove 91 and getting in the Si substrate 90. A fiber end 94 is cut and separated from the fiber 92 by the notch 93. The slanting notch 93 holds a WDM filter 95 in it. A PD 96 is mounted just before the slanting WDM filter 95 above the V-groove 91 formed on the top of the Si substrate 90. A step part 97 is made by cutting the back end of the Si substrate 90. An LD chip 98 lies upon the step part 97. The LD 98 emits 1.3 xcexcmm light 99 for transmitting signals. The transmitting light 99 propagates in the fiber 92, passes through the WDM 95 and reaches the station (not shown). The receiving light 100 of 1.55 xcexcm running in the fiber 92 is reflected from the WDM 95 to be light 101 and sensed by the PD 96. The light path turns upward at the branch.
The module of FIG. 9 seems to have a simple structure, but it is difficult to bury the fiber into the V-groove and to align the LD and the PD with the fiber by sliding the LD and the PD and monitoring the light power sensed by the PD. A single-mode fiber has a core diameter of 10 xcexcm and a cladding diameter of 125 xcexcm. Insertion of the WDM 95 requires the notch to cut the thick cladding. Thus, the reflection loss between the fiber 94 and the fiber 92 increases because of a wide gap between two fibers separated by the WDM at the notch 93. The V-groove becomes so deep that the distance between the WDM 95 and the PD 96 is prolonged. The expansion of light reduces the sensitivity. Further, the light emitted from the LD 98 leaks at the gap, which results in reducing the light entering the optical fiber 92. Hence, the light loss increases.
An LD/PD module of the present invention includes a platform (substrate), a light guide straightly formed at the center of the platform for leading light, a filter laid on the light guide for reflecting a portion of the light and for penetrating another portion of the light, a photodiode mounted on the platform for detecting the light reflected from the filter, and a light emitting source (LD or LED) mounted on the platform at the back of the light guide for generating transmitting-light. Further, there are two forms A and B in the coupling structure between a fiber and and the module.
In the form A, a V-groove formed in the platform fixes the optical fiber. A fiber end is unified to the LD/PD module, which is called a xe2x80x9cpig-tailxe2x80x9d type. The receiving light from the optical fiber moves forward in the light guide (optical waveguide), and is reflected upward from the filter. The reflected light is detected by the photodiode.
In the form B, an optical fiber is connected to an optical connector by inserting guide pins into the optical connector. The optical fiber faces to an optical waveguide of the LD/PD module, as holding the alignment therebetween. This is a receptacle type. The receiving light propagating in the optical fiber of the optical connector advances in the light guide, and is reflected upward by the filter. The reflected light is sensed by the photodiode (PD). The transmitting light generated in the light emitting source (LD or LED) enters the optical waveguide, passes through the filter and goes into the fiber.
The LD/PD module of the present invention differs from the LD/PD module shown by FIG. 9 in which the optical fiber with a diameter of 125 xcexcm is sunk in the V-groove. In this LD/PD module, the light guide can be formed in a shallow surface of the platform so that the reflected light from the filter can go into the PD without spreading over. Hence, the loss of receiving light is small. In the case of fixing the optical fiber in the V-groove, the alignment between the LD and the light guide requires a lot of time and the depth error of the V-groove causes the fluctuation of fiber height. The module of the present invention makes it easy to couple the LD with the light guide endowed with no fluctuation of height. The light guide is a straight waveguide produced by enhancing partially refractive index by doping an impurity to a transparent material of the platform. The light guide is not a fiber glued on the platform but a waveguide formed in the platform by impurity doping. Therefore, it is possible to make the waveguide so shallow. An ordinary waveguide gluing a fiber on the platform requires a depth of more than a half of the diameter of the fiber.
The waveguide on the platform can be made from a transparent inorganic glass material or a transparent plastic material. Quartz glass (SiO2) is the best material for the waveguide. The quartz glass having a low refractive index, e.g., n is about 1.45, which is partially increased by doping Ge and so on. This light guide requires no complex process of producing such a double Y-branch that is shown by FIG. 8, because it is a straight waveguide. When the substrate uses quartz glass, it is enough to allot quartz glass (SiO2) to a part of the substrate, for example, the upper surface of the Si substrate is oxidized to form a SiO2 surface, or a quartz glass waveguide is formed by spattering SiO2 to the surface of the Si substrate.
The LD/PD module of the present invention is applicable to the two-wavelength system using different wavelengths xcex1 and xcex2 for transmitting signals and receiving signals. This two-wavelength system establishes simultaneous bidirectional transmission. In the system, the filter has two roles of reflecting almost all of xcex1 light and of allowing almost all of xcex2 light to pass through. Hence, the filter is a WDM one.
Further, the LD/PD module of the present invention can also use a single wavelength xcex as transmitting and receiving signals. In the single-wavelength system, the filter reflects a part of xcex light and allows the other part of xcex light to pass through at a definite rate.
Since the filter can determine an index of the reflection penetration to constant-wavelength light, the filter can be produced by piling a plurality of dielectric films of different refractive indexes in turn For example, a filter is made by piling a plurality of dielectric films with different thicknesses and different refractive indexes on a glass substrate in turn, or piling a plurality of dielectric films on a transparent polymer material.
The LD/PD module can use an InP substrate and an InGaAs light receiving layer or an InGaAsP light receiving layer as a photodiode. In this case, signal light can use light near infrared light of 1.3 xcexcm, 1.55 xcexcm and so on. The module uses an LD of, e.g., an InGaAsP type laser.
Otherwise, a Si-PD can be used as a photodiode. In the case of the Si-PD, signal light should be visible light with a wavelength between 0.7 xcexcm and 0.8 xcexcm, and a GaAlAs laser can be used as a light source. Both top surface incident-type PDs and bottom surface incident-type PDs can be applicable. Additional mounting of an amplifier capable of amplifying a photocurrent of the PD raises sensitivity, because the amplifier helps the PD to detect weak signals and helps suppressing external noises.
The case (B) is a receptacle-type that has a plurality of guide pins fixed on a platform for connecting the platform to an optical connector, which is explained. The case (B) features in the coupling capable of connecting an end of an optical fiber directly with a light guide. The guide pins work the alignment between the optical connector and the waveguide of the module. The pin diameter, the pin length and the interval between the pins are determined as the optical connector is fitted to the pins, for example, the connector is MT connector, mini-MT connector and so on.
The case (A) is a pig tail-type that has a V-groove formed by digging on a platform for fixing an end of the optical fiber directly to the V-groove, which is explained. There is no alignment between an optical connector and an waveguide.
(1) The first advantage of the present invention is simple in the structure of LD/PD module. The conventional module structure, as shown in FIG. 4, is complicated, large-scaled, and heavy, because individual devices, for example, the WDM module of FIG. 2 and FIG. 3, the LD module of FIG. 5, the PD module of FIG. 6 and so on, are connected to each other by the optical fiber. The module of FIG. 7 unifies the PD with the LD, but requires a large capacity for the light propagating in not the optical waveguide but the space. The light spreads over due to the space-propagating, so that the lens for converging the light is needed.
The prominence of the present invention is certified by comparing the conventional modules. The module of the present invention mounts the WDM, the PD and the LD totally on a sheet of substrate. All necessary devices (LD, PD, WDM and AMP) are mounted on the upper surface of the platform in both the practical mode 1 and mode 2. The structure is extremely simplified. The present invention needs no optical fiber or no optical connector for connecting the devices, for example, PD, LD, WDM and so on. The ratio of space occupied by an excessive case or package is so small that a small-sized and light module is produced. The reliability is high, because the devices are closely mounted. The module of the present invention is so cheap, which brings about a great power for expanding the optical communication widely in ordinary people.
(2) The AMP set up close to the PD can depress external noises.
(3) Both the transmitting light and the receiving light propagate through the same optical waveguide which is formed like a straight line on the substrate surface. The module of FIG. 8 has the optical waveguide formed on the substrate, but the optical waveguide is shaped like a curved line. Such a curved optical waveguide is difficult in producing. This invention enjoys an easy fabrication with a high yield. The PLC (Planar Lightwave Circuit) technology aims at high-performance and sophistication by allotting a WDM function and Y-branch functions to the PLCs in general. This invention does not take such a complicated way but adopts a simple, straight light guide formed on a platform, which alleviates the difficulty of fabrication.
(4) The receiving light propagating through the shallow optical waveguide shoots the slanting filter, turns upward, and goes into the PD. A short optical path prevents the spreading of beams, which reduces the loss of light power. The prior module of FIG. 9 inserts the WDM filter in the slanting upward direction into the optical path, reflects the receiving light upward and introduces the light into the PD. The module of FIG. 9 and the present invention seem to be similar in fabricating the upward filters, but actually there is a great deference therebetween.
In the module of FIG. 9, a thick fiber with a diameter of 125 xcexcm is buried in the substrate surface, and a filter is inserted slantingly into a cross section of the optical fiber. The large filter can not be avoided. The optical path from the filter to the PD is long. The long optical path and no focusing lens aggravate the spreading of beams, which increases the loss of light power. As mentioned before, the shallow light guide shortens the optical path running from the filter to the PD and prevents the expansion of beams, which enables the reduction of light power loss. This is one of the most important advantages.
In the prior module of FIG. 9, the optical fiber must be completely buried in the V-groove. The optical fiber must be fixed in the deep V-groove with an adhesive. The width of the V-groove should be 250 xcexcm to 300 xcexcm in order to bury the optical fiber with a diameter of 125 xcexcm perfectly into the groove. The V-groove is so deep that the filter slit must be deep enough to insert the wide filter. The PD is fixed on the substrate surface as stretching over the V-groove. There occur several troubles in the fixation of the PD, for example, the V-groove is too wide to allot a sufficient area for soldering the PD chip or the PD chip is too small to make the PD chip extend over the V-groove. For instance, when the V-groove has a width of 300 xcexcm, even a 400 xcexcm square PD takes only 50 xcexcm margins for connecting in both sides. This is an insecure condition. If a large 600 xcexcm square PD were used, the connecting margin would be 150 xcexcm, which is too large to bond on the substrate surface. Furthermore, there occurs the problem that the transmitting signal light spreads over until the reflected light enters the PD and the incident light into the PD is greatly decreased.
The present invention forms a shallow optical waveguide on the substrate surface. The PD is easy to be fixed on the substrate due to no groove. A large-sized PD is unnecessary. The distance from the optical guide layer to the platform surface is about 5 xcexcm to 40 xcexcm. The optical path from the filter to the PD is short. The light-coupling with the PD is realized in a short space enough to prevent the beams from stretching over. The efficiency of coupling is high.
(5) This invention saves the process of fixing the fiber in the groove and the process of processing after the fixation. A narrow groove width of the filter is available. It is sufficient for the filter-groove to make a slanting, shallow slit in the substrate. A narrow filter width is enough to satisfy.